While I suspect this will also attract disagreement, I’m going to recommend against anyone doing dialplan like this as well. The context [from-internal-custom] is fine for adding hard coded extension numbers, but the above pattern match will also supersede FreePBX generated dialplan and its usage will have unintended consequences. The correct way to do this is with dialplan hooks.
I will also note that using the SIPAddHeader application is slowly becoming useless as we move more and more to PJSIP. There are better ways of adding headers that are channel driver agnostic.